Tandem type image-forming devices such as color printers are well known in the art. This type of image-forming device includes a plurality of developer units and photosensitive drums and forms color images by sequentially forming toner images in each color on the respective photosensitive drums and subsequently transferring and superimposing the toner images in each color on paper. Conventional tandem image-forming devices require the user to replace the developer units and photosensitive drums for each color, thereby placing a great burden on the user.
One such image-forming device disclosed in U.S. Pat. No. 6,738,590 (corresponding to Japanese patent application publication No. 2003-50531) employs a method of replacing all of the developer units and photosensitive drums together by providing the developer units and photosensitive drums in a single integrated image-forming unit, whereby the image-forming unit is mounted in and removed from the main body of the image-forming device through the top thereof.